Metal structures used in high-performance equipment are often subjected to rigorous operating conditions. For example, various turbine engine components are exposed to significant temperature extremes and degradation by oxidizing and corrosive conditions. Thus, it is common practice in the industry to protect such parts with specialized coatings, such as diffusion coatings and overlay coatings. These coatings are sometimes used in combination with ceramic coatings, e.g., those based on yttria-stabilized zirconia.
In particular, diffusion aluminide coatings are very frequently used to enhance the environmental resistance of the turbine engine components. They are generally formed of aluminide-type alloys, such as nickel-aluminide, platinum-aluminide, or nickel-platinum-aluminide. The coatings are well-known in the art, as exemplified by U.S. Pat. No. 6,042,880 (Rigney et al). They can be applied by a variety of processes, such as pack cementation, above-the-pack deposition, vapor phase deposition, chemical vapor deposition (CVD), and slurry coating processes. Diffusion aluminide coatings typically include two regions or “sublayers”: an additive sublayer which lies on top of the base metal, and a diffusion sublayer below the additive sublayer, which is incorporated into the upper region of the base metal.
In view of the high temperature and harsh operating conditions to which they are sometimes exposed, diffusion aluminide coatings eventually need to be repaired or replaced. Various coating repair methods are sometimes used. For example, the coating can be rejuvenated by certain techniques. As an illustration, the coating surface can be cleaned, and additional coating material can then be applied over the existing coating by one of the deposition processes listed above. Such a technique is advantageous because it tends to maintain the wall thickness of the component. However, after rejuvenation is complete, the coating is sometimes thicker than allowed by dimensional specifications.
Diffusion coating removal and replacement can be required under different circumstances. For example, rejuvenation of a worn or damaged coating may not be possible or beneficial in some instances. Moreover, a coating may have to be removed to permit inspection and possible repair of the underlying substrate.
Coating removal is typically carried out by immersing the component in a stripping solution. A variety of stripping techniques are currently available for removing different types of coatings from metal substrates. The techniques usually must exhibit a considerable amount of selectivity. In other words, they must remove only intended materials, while generally preserving the article's desired structures.
Chemical etching is a popular stripping technique. In such a process, the article is submerged in an aqueous chemical etchant, e.g., one based on one or more strong mineral acids like hydrochloric acid, sulfuric acid, and the like. The metallic coating on the article surface is dissolved as a result of reaction with the etchant.
While chemical etching is effective for a number of situations, it has certain drawbacks. For example, it is often a relatively nonselective process. Thus, in the case of diffusion aluminide coatings, chemical etching tends to remove both the additive sublayer and the underlying diffusion sublayer. Repeated stripping and reapplications of these coatings necessitate repeated removal of the diffusion sublayer. This can undesirably decrease the thickness of the substrate, e.g., a turbine airfoil. Moreover, chemical etching can result in the stripping of coatings from internal passages in the article, which is often undesirable.
Electrochemical stripping processes overcome some of the disadvantages inherent in conventional techniques such as chemical etching. For example, U.S. Pat. No. 6,352,636 describes a very useful electrochemical stripping process. In general, the process selectively removes metallic coatings from the external sections of a metallic article, such as a turbine component.
Nevertheless, additional stripping processes would be welcome in the art. They should be capable of removing substantially all of a given coating, or a selected region of the coating, while not substantially attacking an underlying coating, or a base metal. They should also preserve the structural and dimensional integrity of the base metal, as well as internal passages and cooling holes which may be located within an article of the base metal (e.g., a turbine component).
The stripping processes should not result in the formation of an unacceptable amount of hazardous fumes in the workplace, or produce effluent which cannot easily be treated. Moreover, the new processes should include enhanced process windows, e.g., the time period between the desired removal of selected coating layers and the occurrence of significant damage to other layers or to the substrate. These process windows would provide flexibility and efficiency in a large-scale treatment facility.